Apoptosis, or programmed cell death, typically occurs in the development and maintenance of healthy tissues in multicellular organisms. Apoptotic pathways are known to play a critical role in embryonic development, viral pathogenesis, cancer, autoimmune disorders, and neurodegenerative diseases, as well as other events. Alterations in an apoptotic response has been implicated in the development of cancer, autoimmune diseases, such as systemic lupus erythematosis and multiple sclerosis, and in viral infections, including those associated with herpes virus, poxvirus, and adenovirus.
Caspases, a class of cysteine proteases, are known to initiate apoptosis after they have been activated. Inhibitors of apoptosis proteins (IAPs) are a family of proteins, which contain one to three baculovirus IAP repeat (BIR) domains, namely BIR1, BIR2, and BIR3, and may also contain a RING zinc finger domain at the C-terminus. Examples of human IAPs include, XIAP, HIAP1 (also referred to as cIAP2), and HIAP2 (cIAP1) each have three BIR domains, and a carboxy terminal RING zinc finger. NAIP has three BIR domains (BIR1, BIR2 and BIR3), but no RING domain, whereas Livin and ILP2 have a single BIR domain and a RING domain. The prototype X chromosome linked inhibitor of apoptosis (XIAP) can not only inhibits the activated caspases by direct binding to the caspases, but XIAP can also remove caspases and the second mitochondrial activator of caspases (Smac) through the ubiquitylation-mediated proteasome pathway via the E3 ligase activity of a RING zinc finger domain. The BIR3 domain of XIAP binds and inhibits caspase-9, which can activate caspase-3. The linker-BIR2 domain of XIAP inhibits the activity of effector caspases-3 and -7. The BIR domains have also been associated with the interactions of IAPs with tumor necrosis factor-associated factor (TRAFs)-1 and -2, and to TAB1.
Overall the IAPs function as a ‘constraint’ to apoptosis and may directly contribute to the tumor progression and resistance to pharmaceutical intervention. Interestingly, results demonstrate that resistance to apoptosis can be decrease by siRNA and antisense directed against specific IAP's in the cells. Hence, suggesting that interfering with the activity of the IAP's might prove advantageous in sensitizing disease cells to apoptosis.
A series of endogenous ligands are capable of interfering with IAP-caspase interactions. The X-ray crystallographic structure of XIAP BIR2 and BIR3 reveal a critical binding pocket and groove on the surface of each BIR domain. Two mammalian mitochondrial proteins, namely second mitochondria-derived activator of caspases (Smac) and Omi/Htra2, and four Drosophila proteins (Reaper, HID, Grim, and Sickle), which interfere with IAP function by binding to these sites on their respective BIR domain, have been identified. Each of these IAP inhibitors possesses a short amino-terminal tetrapeptide, AXPY or AVPI-like, sequence that fits into this binding pocket and disrupts protein/protein interactions such as IAP-caspase interactions. Although the overall folding of individual BIR domains is generally conserved, there are alterations in the amino acid sequences that form the binding pocket and groove. As such, binding affinities vary between each of the BIR domains.
A number of compounds have been described, which reportedly bind XIAP including Wu et al., Chemistry and Biology, Vol. 10, 759-767 (2003); United States published patent application number US2006/0025347A1; United States published patent application number US2005/0197403A1; United States published patent application number US2006/0194741A1. Some of the aforesaid compounds, while they appear to target the BIR3 domain of XIAP, may have limited bioavailability and therefore limited therapeutic application. Moreover, the compounds may not be selective against other IAPs and indeed other BIR domains, such as BIR2; this lack of specificity may lead to unexpected side effects.
Thus, IAP BIR domains represent an attractive target for the discovery and development of novel therapeutic agents, especially for the treatment of proliferative disorders such as cancer.